This invention relates to the art of d.c. switching power supplies, and more particularly to a transistor forward converter switching power supply having improved means for returning parasitic energy in the power transformer caused by leakage inductance and magnetizing current to the energy source.
A single transistor forward converter switching power supply includes a transformer which provides a regulated d.c. output voltage in the secondary winding circuit and a transistor switch in the primary winding circuit to which circuit is applied an input d.c. voltage. In such a power supply there must be provided a means to dissipate parasitic energy in the power transformer caused by leakage inductance and magnetizing current. The most common method for doing this has been to provide an additional winding, typically called a dump winding, in the transformer primary. The dump winding, however, limits the B-H excursions of the transformer magnetic core which requires a somewhat larger transformer than if the full capabilities of the core were used. Furthermore, the transformer primary winding and the dump winding must be tightly coupled for good operation and to prevent large voltage spikes in the primary circuit, such tight coupling usually meaning that both windings have the same number of turns. However, when the number of turns on these windings is the same, a practical limitation of below fifty percent is imposed on the duty cycle of the transistor switch in the primary circuit to prevent transistor failure due to transformer saturation. This limitation on the duty cycle, in turn, requires a relatively large number of turns in the transformer secondary winding to maintain a regulated d.c. output with the undesirable results that the transistor switch must handle large currents and rectifiers in the secondary circuit must handle large voltages. In addition, the transistor switch must have a relatively high voltage rating to accommodate voltage spikes which can be additive with respect to input swings in the primary circuit due to coupling requirements between the primary winding and the dump winding.